Not Applicable.
Not Applicable.
1. Technical Field
This invention relates to the field of surveillance and more specifically to the detection of helicopters that potentially represent a threat.
2. Background Art
In the field of electronic surveillance, particularly on the modern battle field, helicopters such as the American Apache helicopter, the European Tiger helicopter as well as undoubtedly Russian and other countries"" helicopters use mast-mounted sights and terrain masking as a way of acquiring a target while remaining undetected. A typical flight scenario would be for a reconnaissance helicopter to fly very low to the ground while approaching a potential target. The helicopter would then expose a minimal portion of itself, such as a mast-mounted sight, which is analogous to observing a surface ship from a submarine. In the case of the helicopter, terrain between the helicopter and the intended target xe2x80x98masksxe2x80x99 the helicopter""s approach.
In the unrelated field of aerodynamics, the operation of a helicopter is fairly well understood. It is an immutable principle of physics that helicoptersxe2x80x94indeed any xe2x80x98heavier-than-air-craftxe2x80x99xe2x80x94can only fly because the airfoils, at any given instant, accelerate a mass of air downward that is at least equal to the mass of the aircraft.
On airplanes, the airfoils (called xe2x80x98wingsxe2x80x99) are bolted firmly to the fuselage at a fixed angle and the entire craft is accelerated along the runway until sufficient xe2x80x98relative airflowxe2x80x99 is generated over the wings that they can deflect a sufficient mass of air to take off. xe2x80x9cLiftxe2x80x9d is the equal and opposite reaction to that downward deflection of the air.
Helicopter airfoils (called xe2x80x98main rotorsxe2x80x99) are rotated about a hub with a feathering hinge at the root, which allows the xe2x80x98angle of attackxe2x80x99 to be increased or decreased, both xe2x80x98cyclicallyxe2x80x99 and xe2x80x98collectivelyxe2x80x99. Because these rotating wings are capable of generating xe2x80x98relative airflowxe2x80x99 solely due to the speed of rotation, it is not necessary for helicopters to have forward speed in order to fly.
But whether we talk about xe2x80x98rotary-wingxe2x80x99 or xe2x80x98fixed-wingxe2x80x99 aircraft, the greater the forward speed with which the aircraft flies through the air, the greater the volume of air per unit of time that the lifting airfoils will act upon. The greater the mass of air deflected, the less vertical acceleration must be imparted to that air mass in order to provide the xe2x80x98liftxe2x80x99 necessary to fly. For example, a crop-spraying airplane flying over a field at only one or two meters above the vegetation will barely rustle the leaves.
On the other hand, slow flying aircraft interact with a smaller volume of air per unit of time and therefore it is necessary to accelerate that air to a greater downward velocity in order to sustain lift. This is the case with a hovering helicopterxe2x80x94particularly a helicopter hovering well clear of the groundxe2x80x94where there is invariably a column of descending air beneath the craft. Hovering a helicopter xe2x80x98out-of-ground-effectxe2x80x99 requires more power than is required for forward flight or hover xe2x80x98in-ground-effectxe2x80x99 and is akin to trying to swim up a waterfall.
Referring to FIG. 1, the vertical velocity of the column of air, also known as the xe2x80x98rotor intakexe2x80x99 region 15, above a hovering or slow moving helicopter 10 depends upon several factors including surface wind, main rotor radius, and xe2x80x98disc loadingxe2x80x99 (that isxe2x80x94the weight of the helicopter divided by the xe2x80x98sweptxe2x80x99 area of the rotor disc). The mass of air entering the rotor intake region is necessarily equal to the mass of air exiting the rotor xe2x80x98down washxe2x80x99 region 16 from the helicopter 10, where helicopter rotor down wash is a fairly well understood phenomenon. Larger helicopters not only have greater mass, but they generally have a higher xe2x80x98disc loadingxe2x80x99 when compared to smaller helicopters. This is because other design influences limit the practical main rotor radius on large helicopters.
We have discovered a means of protecting a potential target by detecting helicopters that are using terrain masking to approach the target. Our invention uses the aerodynamic principles of helicopter flight to detect these helicopters before they have observed the target. Advantageously, our invention reveals the position of the helicopter to the potential target before the helicopter is aware that it has been detected. This invention addresses a long-felt need by ground troops for protection from approaching low flying helicopters.
A robot sentry with a scanning laser observes the sky just above the geographic skyline looking for a vertical airflow pattern characteristic of the rotor inflow to a helicopter rotor. The presence of this vertical airflow pattern indicates the probable presence of a reconnaissance helicopter that is using terrain masking. The robot sentry can be set up to survey the surrounding terrain, using for example a video camera to detect the contrast difference between a darker terrain and lighter sky. The robot sentry can automatically establish an xe2x80x98observation linexe2x80x99 by laser ranging to the geographic skyline or an operator can set the observation line based on local terrain features should as can be determined from a topographic map.
The helicopter is detected by drawing an imaginary line in space, aiming very short duration and small diameter laser pulses at various points along that line, detecting return signals from individual aerosol particles on that line, and correlating an area of vertically descending particles with the area of a helicopter rotor. Once the helicopter is detected, personnel in the area are alerted to the potential threat.